$i=0;
$line = join '', grep { $i++ % 2 ? {s/,/ /g} : 1} split /"/,$line;
##</code><code>##

while(<DATA>) {
  print;
  $i = 0;
  print join '', grep { $i++ % 2 ? {s/,/ /g} : 1} split /"/;
  print "\n";
}

__DATA__
one,two,three
one,"two,three,five",four
"one,two",three
one,two,"three,"

##</code><code>##

one,two,three
one,two,three

one,"two,three,five",four
one,two three five,four

"one,two",three
one two,three

one,two,"three,"
one,two,three